PLASTIDS 



63 



term has been used with special reference to the plastids of algae and also 

 for certain pigmented cells in animals, chromoplast is preferable as a gen- 

 eral term for all colored plastids.^ 



Of all chromoplasts the most familiar is the chloroplasl, whose color 

 is due to the presence of the green pigment chlorophyll. Chloroplasts 

 are usually spherical, ovoid, or discoid in shape, but many bizarre forms 

 are known particularly among the green algse. For example, in (Edo- 



s 



O H 1 J K L 



Fig. 26. — Various types of plastids. A, CEdogonium, showing pyrenoids and stroma 

 starch. B, Leptonema fasciculatum. C, PilayeUa varia. D, Rhodochorton floridulum. 

 E, F, Euastrum dubium, front and side views. G-L, Hyalofheca mucosa, showing division of 

 plastid and pyrenoid with its surrounding starch masses. {From the works of Schmitz, 

 Reinke, Kuckuck, and Carter.) 



gonium the plastid has the form of an irregular net, while in Spirogyra it is 



a coiled ribbon and in desmids a series of radiating plates (Figs. 26, 27). 



The plastid of Anthoceros is spindle shaped and becomes chainlike in 



elongated columella cells (Scherrer, 1914). Chromoplasts are often 



sharply angular, owing in part to the presence of crystals. 



It has been reported for several algae that the chlorophyll is diffused 



throughout the cytoplasm, not being confined to definite plastids. In 



the case of Hydrodictyon, however, Lowe and Lloyd (1928) have shown 



that chloroplasts can be detected by the use of light lying in the region of 



the absorption bands of chlorophyll. 



^A more common practice is to limit the term "chromoplast" to plastids with 

 colors other than green, the chloroplast ("autoplast," Meyer) being placed in a class 

 by itself. For accounts of chromoplasts in this sense, see Rothert (1912) and A. Meyer 

 (1926), 



